The state detection and/or a battery management system of an energy store for hybrid or electric vehicles, for example, is able to supply a number of important items of information to the central engine control, in order to ensure at any time the optimum utilization of the energy store and, at the same time, always the reliable functioning and as long a service life as possible. Such information includes, for instance, the stored and the storable energy, the maximum charging and discharging power, the aging condition (state of health, SOH) or the state of function (SOF). These items of information may be ascertained constantly or at specified intervals. Furthermore, a state detection system is able to take over the equilibration of individual ones of the cells connected up in series or in parallel. For this purpose, as precise as possible an ascertainment of the state of charge (SOC) of a plurality of cells is useful.
In order, for instance, to ascertain the state of charge of a lithium ion cell, for example, the use of various algorithms is believed to be understood. In the simplest case, a charge balancing may be undertaken at this point by integration of the flowing current of the energy store. In this context, physical properties may be drawn upon for the state of charge estimation. For example, the fact may be used that the open circuit voltage (OCV) of an electrochemical cell is often able to demonstrate a clear dependence on the state of charge. In order to enable a dynamic method independent of possible waiting times, additional methods for the estimation may be found. For instance, the current state of charge may be determined by integration of the current. In this context, after a rest phase of specified length, a comparison may take place with a table of open circuit voltage. In the broadening of such a method, one may furthermore connect a simulation model for the cell upstream of the determination of the state of charge from the open circuit voltage. This model may be used to determine the voltage deviation from the open circuit voltage under a load, by modeling the impedance of the energy store.